The properties of photons make them useful in quantum information systems. In particular, photons are easily generated, can be manipulated using well known and inexpensive optical elements, and have quantum states that can maintain coherence during transmissions over large distances. Accordingly, photon states have been employed for communications in many quantum information systems.
Quantum key distribution (QKD) systems are one type of quantum system that can use photon states, and one example of a QKD system uses the well known BB84 process. With the BB84 process, a sending party generates a set a of random bits ai and represents the bits ai using respective photon states (e.g., single-photon states) having polarization encoding based on one of two pairs of orthogonal polarization axes. Conventionally, these two pairs of polarization axes are offset by 45° relative to each other and are respectively referred to as vertical/horizontal and diagonal/anti-diagonal axes. The sending party also generates a second set b of random bits bi that determine whether respective bits ai are represented using photon states with vertical/horizontal or diagonal/anti-diagonal polarization encoding. The receiving party measures the polarization of each photon state corresponding to each bit ai, and for each measurement uses a detector that distinguishes between horizontal and vertical polarizations or a detector that distinguishes between horizontal and vertical polarizations. The detector that the receiver uses for measuring a photon state corresponding to bit ai depends on a corresponding random bit bi′ from a set b′ that the receiving party generates. On average, about half of the bits bi that the sending party used to select the polarization encoding will match the corresponding bits bi′ that the receiving party used for polarization measurement. After the measurements, the sending and receiving parties can exchange sets b and b′, and the parties can separately identify a subset a′ of set a for which the sending and receiving parties happened to used the same polarization bases. Eavesdropping and error rates can be detected by exchanging a portion of the set a′ between the sending and the receiving parties. The remaining portion of set a′ is shared random data that can be kept secret and used as classical encryption keys or for other secure communications purposes. The BB84 process has a high probability of detecting any eavesdroppers, so that if no eavesdroppers are detected, the parties can have a high degree of confidence that the shared random data is secret.
Quantum information systems such as QKD systems using photonic signals must be aligned. In particular, the transmitted photonic signal can be very weak, e.g., a series of single photon states, so the position of the transmitted beam must be precisely aligned with a receiver. Additionally, where polarization encoding is employed, as in a typical application of the BB84 QKD process, the polarization axes of the transmitter and the polarization axes of the detector or detectors in the receiver must be precisely aligned to avoid unacceptable error rates in the polarization measurements. Alignment can be more difficult in some quantum information systems in which at least one of the parties is mobile, in which case, a transmitter and a receiver must be precisely aligned for signal beam position, direction, and orientation when the transmitter and receiver are brought into proximity.
Use of the same reference symbols in different figures indicates similar or identical items.